Dreams are messages projected by your subconscient into your conscient (when you remember them) that talks about an experience that you had, you are having or you'll have. It's a Pscicological definition...

You allways dream but sometimes you don't remember. That's because your subconscient didn't projected the dream into your conscient, but to itself.

Dreams are created by your Highter Self, that tries to teach you, ego, something about your life or giving you a direction to take or warning about something that you should or shouldn't do...

They sometimes are literal, being classified as prophecies, but usually are simbolical, sometimes being classified as prophecies as well, depending on the time the dream talks about. The simbolicals have religious and beliefs implications. They are molded in your beliefs in order for you to understand it easiarly.